When a pressure sensor is used while a sensor chip is directly exposed in a corrosive liquid, corrosion of wirings or pads may be caused. For example, in a pressure sensor used for detecting a differential pressure of an exhaust gas filter of a diesel vehicle or for detecting a pressure in an engine room, pads and wirings made of aluminum (Al) may be easily corroded. Accordingly, in a pressure sensor described in U.S. Pat. No. 5,595,939 (corresponding to JP patent No. 3198779), a corrosion-resistance structure for preventing corrosion of a chip and a wire bonding portion is described.
For example, in a sensor chip J1 of a pressure sensor shown in FIG. 8, an oxidation film J2 is formed on a surface of a semiconductor substrate in which a diaphragm is formed. Furthermore, an aluminum film (Al film) J3 is formed on the oxidation film J2, and an electrical circuit including a gauge resistance formed in the diaphragm of the semiconductor substrate and the Al film J3 are electrically connected through a conductor hole.
Further, a protective film J4 constructed with two layers of a SiN film and a SiO2 film is formed to cover the Al film J3. An opening portion is provided in the protective film J4 so that a predetermined portion of the Al film J3 is exposed from the opening portion of the protective film J4. Furthermore, a metal film J5 made of Ti is formed on the predetermined portion of the Al film J3, and a wire J6 is bonded to the metal film J5 and a terminal pin J7. Accordingly, the terminal pin J7 and the electrical circuit of the sensor chip J1 are electrically connected.
In this pressure sensor, the sensor chip J1 and a connection portion of the wire J6 are placed in an oil J8, and are sealed by a metal diaphragm having a corrosion resistance. Because the sensor chip J1 and the connection portion of the wire J6 are placed in the oil J8, corrosion of those portions can be prevented. However, in this case, sealing members such as an O-ring are also necessary for the metal diaphragm and for sealing the oil J8. Accordingly, the number of components of the pressure sensor is increased, and the size thereof becomes larger.